If you are referring to compute total float, you have 3 options in the schedule options dialog box. Here are the options and there explanations.
Compute Total Float As option
Determines the method to calculate total float for all activities based on the following methods:
Start Float: The difference between the early and late start dates (Start Float equals Late Start minus Early Start).
Finish Float: The difference between the early and late finish dates (Finish Float equals Late Finish minus Early Finish).
Smallest value: Select to use the most critical value, which is the smaller of the start float and finish float values.